When underwater acoustic projectors are driven at levels at which peak acoustic pressures exceed the ambient hydrostatic pressure, a phenomenom known as cavitation can occur. This is manifested by bubbles appearing on or near the surface of the projector and by a sudden reduction in the acoustic loading of the device. Acoustic energy cannot be transferred through the gas bubbles created and consequently the projector ceases to radiate the desired acoustic signal. When the acoustic loading of the projector is reduced, catastrophically high vibrations of the projector can occur with resultant damage to the projector. Further, insidious low level cavitation can cause rapid erosion of the projector face. It is, therefore, highly desirable to avoid cavitation whenever possible. Traditionally this has been achieved by operating the projector at depths where the hydrostatic pressure is high enough to prevent cavitation or by enclosing the projector in an acoustically transparent pressurized container. Unfortunately, there are many instances where the operating depth of a projector is dictated by considerations other than cavitation prevention. For example full power operation at shallow depths may be an operational requirement in order to achieve long distance sound propogation. In the case of hull mounted projectors, such as sonar domes and echo sounders, deep operation obviously is not possible. Enclosing the projector is usually impractical or expensive. Containers capable of withstanding the pressure at, say, 100 m (approx. 1000 k Pa) would have to be made from enormously strong materials. Acoustically transparent materials are not generally strong structural materials. There is, therefore, a need for an alternative method to prevent cavitation around an acoustic projector.